Remove redundant keywords from compiler

Background: #18424

This MR removes redundant do, return, where, etc keywords from the compiler/ code-base.

I felt that this codebase is slightly tricky (due to several factors, ranging from its age to the inherent complexity of a compiler), but I nonetheless took the liberty to fix some other stuff

• Formatting discrepancies: Odd numbers of indentation spaces, like 1 or 7 (??). Some conflicting styles (4 versus 2 indents) in neighbouring code blocks have been harmonised. Some dubious spacing situations are also fixed.
• Monadic code that can be simplified:

-= initCts dflags env' $ do                                                     
-     mapAccumLM (\ccs rhs -> do                                                
-                  (rhs', ccs') <-                                              
-                    coreToTopStgRhs dflags ccs this_mod rhs                    
-                  return (ccs', rhs'))                                         
-                ccs                                                            
-                pairs                                                          
+= initCts dflags env' $                                                        
+    mapAccumLM (\ccs rhs -> swap <$> coreToTopStgRhs dflags ccs this_mod rhs   
+               ccs                                                             
+               pairs                                                           

or

- _ -> do { a <- get bh; return (IfUnpackCo a) }
+ _ -> IfUnpackCo <$> get bh

Moreover, some diffs will appear very large. This is often due to a harmonisation of the indent style because:

• Of a 1-space indented function body between two functions that use 2 or 4;
• A do keyword that was placed on the same line as the rest of the body.

So yes, they will be ugly.

Other than what was described above, no feature change.